1. Field of the Invention
The present invention relates to a method of preparing a polyvinyl alcohol hydrogel, a method of preparing the polyvinyl alcohol hydrogel contained in a plastic container and a Magnetic Resonance Imaging (MRI) phantom made of the polyvinyl alcohol hydrogel contained in the plastic container.
2. Related Art Statement
A process for preparing a polyvinyl alcohol hydrogel by subjecting an aqueous solution of a polyvinyl alcohol to freezing treatment has been known, for example, by Japanese Patent Publication No. 12854/1972. This prior publication discloses that a container and an aqueous solution of a polyvinyl alcohol contained in the container are subjected to freezing treatment at a temperature below - (minus) 5.degree. C. However, as described in this publication, a satisfactory polyvinyl alcohol hydrogel cannot be formed unless the temperature is lowered within the range of - (minus) 15.degree. C. and - (minus) 50.degree. C. In addition, the gel prepared by this known process is relatively soft and weak.
Another process has been known by Japanese Laid-open Patent Application No. 130543/1982, in which an aqueous solution of a polyvinyl alcohol is frozen and the frozen mass is then dehydrated to prepare a gel having high strength. In this known process, an aqueous solution of a polyvinyl alcohol is contained in a container and frozen and molded at a temperature of lower than - (minus) 6.degree. C. to obtain a molded mass, which is dried in vacuum without thawing the same to prepare a gel having high strength. However, in practice, as will be seen from the Examples and the description in the Publication, a molded mass is prepared by freezing the aqueous solution at a relatively lower temperature of from - (minus) 20.degree. C. to - (minus) 80.degree. C. prior to the drying in vacuum.
On the other hand, Japanese Laid-open Patent Application No. 247440/1986 discloses a phantom used for diagnosis by MRI, which is made of a gel prepared by cooling to freeze an aqueous solution of a polyvinyl alcohol at a temperature of below - (minus) 10.degree. C., followed by thawing, additional freezing-thawing cycles being repeated by 2 to 7 times.
Japanese Laid-open Patent Application No. 153056/1988 discloses a phantom provided with at least one sample receiving port, which is prepared by charging an aqueous solution of a polyvinyl alcohol into a plastic container and subjecting the aqueous solution in the container to freezing and thawing treatment.
Although these prior publications disclose prominent technology for preparing polyvinyl alcohol gels, and disclose a phantom made of a polyvinyl alcohol gel and used for MRI diagnosis or disclose that the polyvinyl alcohol hydrogel prepared by the freezing-thawing cycles has appreciably superior properties over those of the known phantoms for MRI diagnosis, freezing must be effected at a low temperature of below - (minus) 10.degree. C. in the practical working of the processes, leading to expansion of the frozen mass during the freezing step to cause various problems.
In detail, when an aqueous solution of a polyvinyl alcohol is contained in a plastic container and frozen at a temperature defined by the known processes, the container is broken as will be described hereinafter in the Comparative Examples. When the plastic container described in the above mentioned Japanese Laid-open Patent Application No. 153056/1988 is used, a considerably large empty space must be left in the upper portion of the plastic container, and the expanding polyvinyl alcohol gel rises into the empty space to form irregular surface, and in addition, strain or stress is developed throughout the frozen mass. Accordingly, a further improvement is required to prepare a gel having dimensional stability. A still more improvement is demanded for preparing a phantom for MRI diagnosis in which accuracy in dimensions is required, as will be described in detail in the Comparative Example.
On the other hand, as will be described in detail in the comparative Example, when an aqueous solution of a polyvinyl alcohol is not brought to a low temperature of below - (minus) 10.degree. C. but is cooled to a moderate cooling temperature of about - (minus) 5.degree. C., a jelly-like gel is formed and the gel is returned back to an aqueous solution as the temperature is raised to room temperature, and thus a polyvinyl alcohol gel cannot be formed.